1. Field of Invention
This invention relates to laminated capillaries which are used for interfacing higher pressure ionization sources to lower pressure ion destinations such as mass spectrometers, ion mobility spectrometers, and ion beam targets.
2. Background—Description of Prior Art
Dispersive sources of ions at or near atmospheric pressure, such as, atmospheric pressure discharge ionization, chemical ionization, photoionization, or matrix assisted laser desorption ionization, and electrospray ionization, generally have low sampling efficiency through conductance or transmission apertures and capillaries or tubes. Less than 1% [often less than 1 ion in 10,000] of the ion current emanating from the ion source is detected in the lower pressure regions of the present commercial interfaces for mass spectrometry.
FIG. 1 show simulated trajectories of ions approaching a capillary entrance from a 400 V/mm ion source region into the relatively field-free inner channel of a capillary. A viscous [gas] flow velocity component is added to these ions in the direction of the capillary flow. This simulation shows the electric field penetration from the source region creates significant dispersion of ions and loss of ions to the walls at the inlet of a capillary. The losses of ions to walls will generally have two consequences; first, in the case of conducting [metal] capillaries, the ions will give up charge (usually through a redox process) or, second, in the case of dielectric materials [glass] the ion will accumulate on the surface and further retard introduction of subsequent ions into the flow through the capillary. Either way, the ions are primarily lost at or slightly downstream of the entrance of the capillary tube.
U.S. Pat. No. 4,542,293 Fenn et al. (1985)1 demonstrates the utility of utilizing a dielectric [glass] capillary with metal ends with a large electric potential difference along the axis of the capillary, referred to capacitive charging, to transport gas-phase ions from atmospheric pressure to low pressure where the viscous forces within a capillary push the ions against a electrical potential gradient. This technology has the significant benefit of allowing grounded needles with electrospray sources. Unfortunately, this mainstream commercial technology2 transmits only a fraction of a percent of typical atmospheric pressure generated ions into the vacuum. The majority of ions are lost at the inlet of the capillary due to the dispersive electric fields, at the inlet, dominating the motions of ions (FIG. 1).
The requirement for capacitive charging of the dielectric tube for the transmission of ions, as well as, the acceptance or entry of ions into the capillary, is highly dependent on the charges populating the inner- and outer-surface of the capillary. This dependence of surface charging limits the acceptance and transmission efficiencies of Fenn et al.'s technology. Contamination of the large surface area of the inner-walls of the capillary from condensation, ion deposition, particulate material or droplets can change the surface properties and therefore reducing these efficiencies. In addition, since a large amount of energy is stored within the capillary, contamination can lead to electrical discharges and damage to the capillary, sometimes catastrophic. Therefore, care must also be taken to keep the inner- and outer-surfaces clean and unobstructed, presumably in order not to deplete the image current that flows on the outer-surface of the dielectric or the current that flows along the inner-surface.
Examples of metal capillaries are disclosed—for example, in U.S. Pat. No. 4,977,320 to Chowdhury et al. (1990)3, and U.S. Pat. No. 6,583,408 B2 (2003)4 and in U.S. patent application publication 2002/0185559 A1 (2002)5 both to Smith et al. Chowdhury et al. and Smith et al. both demonstrated the use of heated metal capillaries to both generate and transmit ions into the vacuum. The efficiencies of these devices are low as well. This technology samples both ions and charged droplets into the capillary where, with the addition of heat, ion desorption is facilitated. Drops undergoing coulomb explosions inside of a restricted volume of the lumen of the capillary will tend to cause dispersion losses to the walls were the charges are quickly neutralized and: not resulting in the surface charging up. But similar to Fenn et al.'s dielectric capillary, this technique suffers the same limitation from losses at the inlet due to the dispersive electric fields (FIG. 1), as described above.
Lin and Sunner (1994)6 studied a variety of effects on transmission through tubes of glass, metal, and Teflon. A wide variety of parameters were studied including capillary length, gas throughput, capillary diameter, and ion residence time. Effects from space charge, diffusion, gas flow, turbulence, spacing, and temperature where evaluated and discussed. Some important insights where reported with respect to general transmission characteristics of capillary inlets. However, they failed to identify field dispersion at the inlet as the first step in the loss of ions. In the case of glass capillaries, this dispersion and eventual impact of the ions on the inner-surfaces of capillary lumen leads to charging of the inner-surface of the capillary lumen at the entrance of the capillary preventing ions from entering into the capillary.
Several approaches have been proposed to eliminate or reduced the charging of the surfaces at the entry of glass or dielectric capillaries—for example, in U.S. Pat. No. 5,736,7407 (1998) and U.S. Pat. No. 5,747,7998 (1998) both to Franzen, U.S. Pat. No. 6,359,275 B1 to Bertsch et al. (2002)9; and U.S. Pat. No. 6,486,469 B1 (2002)10 and U.S. Pat. No. 6,583,407 B1 (2003)11, and U.S. patent application publication 2003/003452 A1 (2003)12 all to Fischer et al. Franzen (U.S. Pat. No. 5,736,740) proposes the use of a highly resistive coating on the inner surfaces of the capillary tube or use capillaries that are themselves highly resistive, such as, glass capillaries, to prevent charge accumulation as a means to facilitate the focusing of ions toward the axis of the capillary. Although it is difficult to distinguish this art from Fenn et al. (U.S. Pat. No. 4,542,293), in that the glass tubes in both approaches are highly resistive [or weakly conducting dielectrics], Franzen does argue effectively for the need to control the inner surface properties and therefore the internal electric fields. Irregardless, Franzen's approach will suffer from the same limitations as Fenn's, that is loss of ions in the dispersive electric fields at the inlets of capillaries and apertures.
Bertsch et al. (U.S. Pat. No. 6,359,275 B1) proposes a similar approach to Franzen to prevent charging of the surface by coating the inner-surface. But unlike Franzen, Bertsch et al. coats the inner-surface of the capillary near the capillary entrance with a conductive material, thereby bleeding away any charge that builds up on the inner-surface to the end-cap. Bertsch et al. eliminates surface charging while still keeping the benefits of the dielectric tube transport in the nondispersive region [downstream region] of the capillary. This approach addresses the problem of charge accumulation on the inner-surfaces, but it does not remove the significant losses of ions at the inlet due to dispersion (FIG. 1). Again, suffering the same limitations of Fenn et al.'s, Franzen's, and Chowhdury et al.'s devices—lose of ions at the inlet due to dispersive electric fields.
Franzen (U.S. Pat. No. 5,747,799) and W.O. patent 03/010794 A2 to Forssmann et al. (2002)13 addresses the need to focus ions at or into the Inlet of capillaries and apertures in order enhance collection efficiencies by the use of a series of electrostatic lens at or in front of the inlet. In Franzen's device the ions are said to be first, attracted to the inlet by electrostatic potentials and once in the vicinity of the inlet the ions are entrained into the gas flowing into the tube or aperture by viscous friction. This invention fails to account for the dominance of the electric field on the motion of ions in the entrance region. At typical flow velocities at the entrance of tubes or apertures, the electric fields will dominate the motion of the ions and the ions that are not near the capillary axis will tend to disperse and be lost on the walls of the capillary or aperture inlet. With this device, a higher ion population can be presented to the conductance opening at the expense of higher field ratios across the aperture or along the capillary but at the expense of higher dispersion losses inside the aperture or tube.
Forssmann et al. (03/010794 A2) describes a series of electrodes, or funnel optics, upstream of the capillary inlet in order to concentrate and direct ions toward or into the capillary inlet. This approach utilizes funnel optics in front of an electrospray source in order to concentrate ions on an axis of flow by imposing focusing electrodes of higher electrical potential than the bottom of the so called accelerator device, the first electrode in the series. This device frankly will not work. The ions formed by the electrospray process will be repelled by this funnel optics configuration and little to no transmission of ions to the aperture or capillary inlet will occur. Most of the inertial energy acquired by the ions in the source region is lost to collisions with neutral gas molecules at atmospheric pressure; consequently the only energy driving the ions in the direction of the capillary inlet or aperture will be the gas flow which under normal gas flows would be insufficient to push the ions up the field gradient imposed by the funnel optics. This device does not operate in fully developed flow as will be described in the present invention.
U.S. Pat. No. 6,486,469 B1 (2002) and U.S. Pat. No. 6,583,407 B1 (2003); and U.S. patent application publication 2003/003452 A1 (2003) to Fischer et al. all utilize external electrodes and butted dielectric tubes to provide enhanced control of the electric fields within the capillary. While Fischer et al. (U.S. Pat. No. 6,583,407 B1) utilize the conductive coating proposed by Bertsch et al. (U.S. Pat. No. 6,359,275 B1) to eliminate surface charging, all three devices do not address issues related to inlet losses due to dispersive electric fields at the inlets of capillaries and apertures, as presented in FIG. 1. In addition, all these devices still utilize significantly large dielectric inner-surfaces with the associated problems with surface charging, contamination, and discharge.
References
    1 Fenn, J. B., Yamashita, M., Whitehouse, C., “Process and apparatus for changing the energy of charged particles contained in a gaseous medium,” U.S. Pat. No. 4,542,293 (Sep. 17, 1985).    2See Analytica of Branford, Branford, C T, http://aob.com; Agilent Technologies, Wilmington, Del., http://agilent.com/chem; and Bruker Daltonics, Billerica, Mass., http://www.bdal.com.    3 Chowdhury, S. K., Katta, V., Chait, B. T., “Electrospray ionization mass spectrometer with new features”, U.S. Pat. No. 4,977,320 (Dec. 11, 1990).    4 Smith, R. D., Kim, T., Tang, K., Udseth, H. R., “Ionization source utilizing a jet disturber in combination with an ion funnel and method of operation,” U.S. Pat. No. 6,583,408 B2 (Jun. 24, 2003).    5 Smith, R. D., Kim, T., Udseth, H. R., “Ionization source utilizing a multi-capillary inlet and method of operation,” U.S. Patent Application Publication 2002/0185595 A1 (Dec. 12, 2002).    6 Lin, B., Sunner, J., “Ion transport by viscous gas flow through capillaries”, J. Am. Soc. Mass Spectrom. 5, pages 873-885 (1994).    7 Franzen, J., “Method and device for transport of ions in a gas through a capillary,” U.S. Pat. No. 5,736,740 (Apr. 7, 1998).    8 Franzen, J., “Method and device for the introduction of ions into the gas stream of an aperture to a mass spectrometer,” U.S. Pat. No. 5,747,799 (May 5, 1998).    9 Bertsch, J. L., Fisher, S. M., Riccomini, J. B., “Dielectric conduit with end electrodes”, U.S. Pat. No. 6,359,275 B1 (Mar. 19, 2002).    10 Fisher, S. M., Russ, C. W., “Dielectric capillary high pass ion filter”, U.S. Pat. No. 6,486,469 B1 (Nov. 26, 2002).    11 Fisher, S. M., Russ, C. W., “Method and apparatus for selective ion delivery using ion polarity independent control”, U.S. Pat. No. 6,583,407 B1 (Jun. 24, 2003).    12 Fisher, S. M., Russ, C. W., “Dielectric capillary high pass ion filter”, U.S. Patent Application Publication 2003/0034452 A1 (Feb. 20, 2003).    13 Forssmann, W-G, John, H., Walden, M., “Mass Spectrometry Device,” WO Patent 03/010794 A2 (Feb. 6, 2003).